Transmit and receive antennas are now being used on the exterior surfaces of commercial aircraft to provide broadband interconnectivity with ground based stations via one or more satellite-based transponders. Such antennas are often electronically scanned phased array antennas; mechanically augmented phased array antennas; mechanically scanned reflector antennas summarized herein as antennas from the group including single reflector antennas, reflector arrays or planar arrays configured in a planar or elliptical shape; or other forms of antennas which are disposed on an exterior surface of the fuselage of the aircraft. Except for non-mechanically scanned phased array antennas, the antenna is typically mounted within a radome and radiates its beam through the radome when in a transmit mode of operation. Non-mechanically scanned phased array antennas normally do not require a radome due to their low aerodynamic cross section.
An undesirable consequence of mounting the antenna within an aerodynamically shaped radome is the creation of reflections of electromagnetic energy caused by the radiated electromagnetic energy impinging the radome at angles other than normal to the interior surface of the radome. However, when electromagnetic energy impinges the radome at an angle normal to the surface of the radome, the great majority of the energy passes through the radome. A mechanically scanned antenna system is required to point the transmit-receive antenna beam over 360 degrees in azimuth and nearly 90 degrees in elevation during aircraft to satellite communications operation. Aerodynamic radomes are frequently designed with multilayered dielectric walls to minimize the loss of transmit and receive electromagnetic energy passing through the radome. The radome design performs effectively for antenna radiated energy angles within plus or minus 50 degrees from normal incidence to the interior surface of the radome. However, as the angles of incidence increase from 70 to 90 degrees, reflection losses increase significantly. Interior radome surface reflections are highest near the radome wall transition from vertical to horizontal, when the antenna system is pointed at elevation angles from 0 to 30 degrees, and in particular, aft towards the tail of the aircraft. This is due to the common flattened teardrop shape used for the aerodynamic radome, which tapers both in width and in height towards the aft direction. In this region, the electromagnetic energy emanating from the antenna impinges on the wall of the radome at incident angles ranging from 80 to 90 degrees. Reflected energy is highest at these large angles of incidence.
The problem with reflected energy is also acute when the main beam from the mechanically scanned antenna is scanned along an axis which is close to parallel to the exterior of the fuselage of the aircraft. At this scan angle, the electromagnetic energy impinges an interior surface of the radome which is tapering toward the fuselage. Electromagnetic energy impinges the interior surface at an angle which is not normal thereto, thus causing a significant degree of energy to be reflected by the interior surface of the radome back toward the fuselage.
Reflected energy is highly undesirable as this energy can be directed into the skin of the aircraft, wherein the skin can act as an antenna to further radiate the energy towards other RF receivers or transceivers in the vicinity of the aircraft, and particularly transceivers located on the ground below the aircraft. It is also undesirable for a communications system to have its high level radiated transmit power reflected back into the antenna aperture and into the low noise receiver. Since the radome must have a highly aerodynamic shape, it becomes impossible to avoid the problem of reflections within the radome because at such angles as described above, the main beam radiated by the antenna will always be impinging the walls of the radome at angles that are not normal to the interior surface of the radome.
Accordingly, it would be highly desirable to provide some form of attenuation apparatus within the radome which at least partially circumscribes the antenna to reflect and/or absorb a portion of the radiated electromagnetic energy from the antenna toward the interior surface of the radome such that the reflected electromagnetic energy is absorbed or impinges the interior surface of the radome at an angle normal thereto, thus minimizing the reflections that occur within the radome when the antenna is scanned.
It would also be highly desirable to provide such an attenuation apparatus as described above that does not interfere with operation of the antenna, whether the antenna is a mechanically scanned phased array antenna, a mechanically scanned antenna including reflectors, reflector arrays or planar arrays, or other form of reflector antenna, and further which does not require modifications to the shape of the radome or necessitate non-aerodynamic modifications to the contour of the radome.
It would also be highly desirable to provide additional attenuation of reflected energy within a radome perimeter where an attenuation apparatus cannot totally shield the fuselage surface under the radome.